1. Field of the Invention
The present invention relates to integrated circuit fabrication techniques. In particular, the present invention relates to copper-aluminum metallization.
2. Discussion of the Related Art
In the past, pure aluminum metallization had been used in integrated circuits because of the low cost, good ohmic contact, and high conductivity of aluminum. More recently, aluminum alloys that provide advantages over pure aluminum have also been developed. For example, aluminum is alloyed with copper to provide an aluminum alloy with improved electromigration resistance. However, increasing circuit densities and faster operating speeds of integrated circuit technology require the use of metals having higher conductivities than that of aluminum.
One example of a metal with a conductivity higher than that of aluminum is copper. In the fabrication of an integrated circuit device utilizing pure copper metallization, copper is deposited into vias, trenches, or other recesses to interconnect semiconductor devices or conductive layers formed on a semiconductor substrate. For example, FIG. 1 illustrates a cross-section of a recess 18 in a portion of a prior art copper metallization structure 10. Referring to FIG. 1, in copper metallization structure 10, a copper wire 16 is used to interconnect two or more conductive regions formed in a substrate. Copper wire 16 is formed in a recess 18 defined in a dielectric layer 12. Dielectric layer 12 is typically formed out of silicon dioxide. A diffusion barrier 14 which is formed out of a metal such as tantalum (Ta), Tungsten (W), Chromium (Cr) or a metal composite such as titanium-nitride, titanium-tungsten, tungsten-nitride, or tantalum nitride, is provided as both a diffusion barrier 14 and as an adhesive layer which improves the adhesion of copper wire 16 to dielectric layer 12.
A standard technique such as physical vapor deposition or chemical vapor deposition can be used to form diffusion barrier 14 and copper wire 16 in recess 18. However, physical vapor deposition of copper has poor step-coverage, resulting in void or seam formation. Further, a chemical vapor deposition technique requires careful control of selectivity, processing temperatures, and incurs high capital costs. As a result, electroless copper deposition has been suggested as a superior alternative technique for fabricating copper metallization. This is because electroless deposition techniques incur less capital costs, provides high quality deposited films, is inherently selective and is a conformal deposition process.
One example of electroless copper deposition is described in copending patent application Ser. No. 08/887,264 filed Jan. 16, 1996. Electroless copper deposition, which proceeds at relatively low temperatures, can be used to fill vias, trenches, or other recesses in dielectrics, and to fabricate in-laid copper metallization. Moreover, electroless copper deposition also offers advantages of low cost, high throughput, high quality electroless copper films, and superior recess filling capability.
However, pure copper metallization also has undesirable qualities that present integrated circuit reliability problems. For example, without proper measures, copper is easily oxidized and diffuses into silicon and silicon oxide, causing device failure. Also, pure copper does not adhere well to dielectrics such as silicon dioxide, and presents corrosion problems. As a result, aluminum has been suggested as a doping material to be introduced into the copper metallization to improve oxidation resistance and to act as a diffusion barrier. (See P. J. Ding, W. A. Lanford, S. Hymes, and S. P. Murarka, "Effects of the addition of small amounts of Al to copper: Corrosion, resistivity, adhesion, morphology, and diffusion," Journal of Applied Physics, Volume 75 Number 7).
Fabricating copper-aluminum metallization can be accomplished using a variety of well known techniques. For example, a copper/aluminum bilayer can be deposited by sputtering. Alternatively, a copper-aluminum alloy film can be obtained by annealing an aluminum film provided above or below a copper film. In a conventional process, an annealing step and a wet etch step are required after the copper/aluminum bilayer or the copper-aluminum alloy is formed. Thus, a process for fabricating copper-aluminum metallization based on electroless copper deposition is desired.
Unfortunately, an electroless copper deposition process for fabricating copper-aluminum metallization has proved to be extremely difficult to develop. One difficulty is that co-deposition of aluminum with copper cannot be achieved by electroless plating, because the electrochemical potential of aluminum reduction is highly negative. Another difficulty is that electroless copper deposition on aluminum films cannot be achieved because aluminum films dissolve in a basic electroless copper solution (pH&gt;11).
Accordingly, it would be desirable to provide an improved method for fabricating copper-aluminum metallization using electroless copper deposition.